The present invention relates to a developing device for use in an image forming apparatus based on electrophotographic copying process, in which two-component developer composed of toner and carrier is used.
Conventionally, a method of performing multi-color copying is known. In this known method, an image forming apparatus in which a plurality of developing devices are provided at a side of an electrostatic latent image support member is employed such that a multi-color copy is obtained by overlapping on a transfer paper sheet, toner images formed in different colors. In the known method, in the case where an original document of three colors is copied, three copying operations are performed and thus, drive of the developing devices is required to be changed over for the colors, respectively.
Meanwhile, another prior art multi-color image forming method employs an image forming apparatus in which a plurality of developing devices are provided at a side of an electrostatic latent image support member such that the developing devices are selectively driven during one copying operation. In this prior art method, for example, a first region of the original document ranging from a front edge of the original document to a designated point is reproduced in a first color and then, a second region of the original document ranging from the designated point to a rear edge of the original document is reproduced in a second color.
In the case of the above described known multicolor image forming methods, especially, the latter one, image density at a boundary portion of the regions should be raised to a normal level instantaneously in each of the developing devices. The reason is that if a long time period is required, after start of a subsequent one of the developing devices, for stabilizing density of an image to be formed by this developing device, density of a front end portion of the corresponding copying region of the original document becomes low or non-uniform, thereby resulting in deterioration of image quality.